1. Field of the Invention
The present invention relates to a heterojunction field effect transistor (FET) applied to a millimeter wave or microwave transmitter-receiver system and a high speed digital circuit.
2. Description of the Related Art
FIG. 9 shows the structure of a conventional heterojunction FET. Such heterojunction FET was reported, for example, in "IEEE Electron Device Lett.," Vol. EDL-13, p. 325, 1992, by T. Akazaki.
In the figure, the heterojunction FET consists of a semi-insulating (hereinafter called "SI") InP substrate 10, an undoped InAlAs layer 91 constituting a buffer layer, an undoped InGaAs layer 92 constituting a channel layer, an undoped InAlAs layer 93 constituting a spacer layer, a Si planar-doped layer 94, an n-type InAlAs layer 95 constituting an electron supply layer, an undoped InAlAs layer 96 constituting a Schottky layer, and cap layers 97 and 99 which are constituted by an n-type InAlAs layer and an n-type InGaAs layer, respectively, a source electrode 11S and a drain electrode 11D formed by vacuum deposition on the n-type InGaAs cap layer 99 to make ohmic contact with the channel layer 92. In a region between the source electrode 11S and the drain electrode 11D, a gate electrode 12 is formed by vacuum deposition on the surface of the Schottky layer 96 which is exposed by etching out parts of the epitaxial layers.
FIG. 10 shows a conduction band profile between the n-type InGaAs cap layer 99 and the undoped InAlAs buffer layer 91 of such heterojunction FET. Because such a heterojunction FET has the n-type InAlAs layer 97 between the undoped InAlAs Schottky layer 96 and the n-type InGaAs cap layer 99, the potential barrier caused by discontinuity of the conduction band at the interface between the InAlAs Schottky layer 96 and the InAlAs cap layer 97 becomes parabolic. Thus, when compared with a case where the n-InGaAs layer 99 is formed in direct contact with the undoped InAlAs layer 96, the effective thickness of the potential barrier is reduced, and the tunnel current is caused to easily flow through the InAlAs layer forming effectively an ohmic contact without alloying, i.e., by forming a non-alloy ohmic contact.
As discussed above, the heterojunction FET of the prior art can form non-alloy ohmic contacts by providing a cap layer having a dual-layered structure of n-InGaAs/n-InAlAs. However, because it has conduction band discontinuity as high as about 0.5 eV at the InAlAs/InGaAs hetero-interface, a high potential barrier is formed at the interface between the InAlAs layer 97 and the InGaAs layer 99 so that the contact resistivity (.rho..sub.c) between the cap layer and the channel layer cannot be sufficiently lowered.
Generally, contact resistance (R.sub.c) of a heterojunction FET having a cap layer with low resistance is expressed by using sheet resistance (r.sub.s) and .rho..sub.c, as follows: EQU R.sub.c =(r.sub.s .rho..sub.c).sup.0.5 cot h(d/L.sub.T) (1)
Here, d is the length of electrode, and L.sub.T =(.rho..sub.c /r.sub.s).sup.0.5 is the transfer length. Generally, because d is sufficiently larger than L.sub.T, equation (1) reduces to R.sub.c .apprxeq.(r.sub.s .rho..sub.c).sup.0.5. Thus, an increase of .rho..sub.c in the ohmic electrode leads to increase of R.sub.c which increases the source resistance and the drain resistance to degrade power gain and noise factor.
It is known in the art that, as the impurity concentration in a semiconductor becomes high, adjacent impurity atoms start to affect each other so that discrete impurity levels converge to form degenerate impurity bands having a band-like nature. In the n-type InGaAs, carrier degeneration starts at the impurity concentration of 5.times.10.sup.17 cm.sup.-3 or more to form an impurity band. Thus, in a conventional heterojunction FET in which the n-InGaAs cap layer 99 is provided in contact with the n-InAlAs cap layer 97, even though an electron accumulation layer is formed near the hetero-interface of the two n-type cap layers, the Fermi level is pinned by an impurity band with high density of states in the n-type InGaAs layer 99 making band bending at the interface small so that the potential barrier in the n-type InAlAs layer 97 cannot be lowered sufficiently.